The present invention generally relates to radio communications systems and more particularly pertains to a radio communications system carrying data messages between remote data terminal units and fixed stations. In many radio data communications systems, messages are generated at random without coordination between remote data units. This type of system is similar to the conventional two-way radio voice communication in extensive use in the domestic land mobile service and which results in the lowest cost service available to its users. However, the lack of coordination between remote data units occasionally results in two data units attempting to transmit a message at approximately the same time thereby contending for a radio channel.
Radio channel contention is well known to those skilled in the art. Solution of this problem has taken the form of randomizing transmissions and checking for channel activity before transmissions. These solutions, however, can add delay which reduces the number of data messages which can be carried by the channel.
Receiver selection techniques, which enable one radio receiver having a better signal than others receiving on the channel to be chosen to receive a message, consider a problem similar to that solved by the present invention. A receiver is generally selected on the basis of a signal-to-noise comparison or a tone identification and this selection is accomplished by a central controlling function. Receiver selection, however, does not necessarily help to resolve an essentially simultaneous transmission by two radios.
Controlled systems, such as a cellular radio telephone system or a trunked dispatch system (further described in U.S. Pat. NO. 4,012,597 assigned to the assignee of the present invention), utilize a channel assignment technique in which a remote unit must request permission to transmit a message. Unless this permission is given, the remote unit may not transmit its message and channel contention is limited to the requesting channel where requests are short and may be repeated. In a system where the data messages are short and of a burst nature, the set-up time of these controlled systems is typically longer than the message and therefore wastes channel time. Also, these systems may add complexity and expense to what is desired to be a low cost system.
The majority of radio channels dedicated to land mobile communications service in the United States employ frequency modulation (FM) as the method of modulating the radio carrier with message information. One fundamental advantage of FM radio transmission is the capture effect, which is a quality inherent in an FM receiver to enhance a stronger signal and diminish a weaker signal when both are present on the same channel. Thus one signal which is slightly stronger than a second signal on the same channel will be heard clearly and the second signal may not be heard at all.
The reception at a fixed station of one signal in the presence of weaker interfering signals is fundamental to the efficient and economic operation of radio data communication systems. The long term practicality of large area single channel systems as competitive generic products is in fact dependent upon the successful reception of strong signals in the presence of weaker but still decodable signals.
Contention among remote data unit transmissions on the radio channel can result in an excessive number of lost messages. The possibility of channel contention collision among remote data units transmitting "quasi-simultaneously" makes it desirable to attempt to salvage the stronger of two interfering signals.
When a weaker signal begins during the reception of a stronger signal, the fixed station takes no special action. But when a strong signal begins during the transmission of a message via a weaker signal, the fixed station controller must detect this and begin processing the new signal since it will capture and obliterate the original.
In order to deliniate the beginning of a data message, each message in a typical data system begins with a specific synchronization bit sequence. The radio channel is continuously monitored for a sequence that is nearly identical to the synchronization pattern and the data which is subsequently received is framed by this sync pattern. However, on the order of one in one thousand messages will, by random chance, contain a sequence that matches the sync sequence closely enough to spuriously indicate the beginning of a new message. Because of this, sync sequences that occur within a message are normally ignored, since the message would otherwise be inherently undeliverable, in that the original and all repeat transmissions would be aborted when the false sync sequence was reached.
Thus the quandary exists when a sync sequence occurs during a message reception: should the message continue to be decoded or should a new message be framed and decoding commenced. The consequence of a wrong decision is an unnecessarily lost data message.